Gas Chromatography—Mass Spectrometry (GCMS) is a powerful analytical technique first commercially developed in the late 1960's but which has to some extent been eclipsed by the more recent development of liquid chromatography-mass spectrometry as a routine analytical tool. Nevertheless, in view of its high sensitivity and relative ease of use, GCMS is a valuable method of analysis of compounds amenable to separation by gas chromatography.
Many prior GCMS instruments employ ionization sources operable at sub-atmospheric pressure, for example electron impact or chemical ionization sources in which ionization of analyte molecules takes place at a pressure typically less than about 2 torr. At least in the case of packed column gas chromatography where a carrier gas flow rate of more than about 5 ml/minute is employed, a mass-selective separation device such as a jet separator is typically employed. This selectively removes the low mass carrier gas (typically helium) from the column effluent while transmitting a greater proportion of the higher mass analyte molecules in a reduced flow which the mass spectrometer can tolerate. Alternatively, in the case of capillary column gas chromatography, where a flow rate of less than 1 or 2 ml/minute is usually employed, the effluent from the column may be conveyed directly into the ion source of the spectrometer through a capillary restrictor, frequently the chromatographic column itself. If necessary a simple flow splitter may be used if the column flow rate is too great.
In most cases, the separator, capillary restrictor and all other components of the transfer system are heated to minimise analyte losses by condensation.
If a separator or flow splitter is employed, substantial loss of sample is inevitable. Further, the ionization processes involved in electron impact and (to a lesser extent) in chemical ionization sources are relatively violent and often result in excessive fragmentation of labile analytes, even those which can be successfully separated by gas chromatography. Experience with the use of atmospheric pressure ionization sources in liquid chromatography—mass spectrometry has shown that such sources induce far less fragmentation, so their use for GCMS is an attractive proposition, both from the standpoint of lower fragmentation and the lack of any flow splitting requirement. However, despite reports describing such sources first appearing in the 1970's, the technique has not so far met expectations, particularly in respect of sensitivity.